1. Field of Invention
The present invention relates to vegetable protein products and, in particular, to novel lipoprotein products derived from wheat protein, as well as processes for the production of such products, and their use.
2. Description of the Prior Art
Vegetable protein products such as soy protein in its various forms (flour, concentrate, etc.) and wheat proteins, especially vital wheat gluten, find many applications in food and other areas. Of special importance, because of the unique properties imparted by its vital or functional nature, is gluten.
Gluten is a concentrated natural protein generally taking the form of a light tan powder having a relatively bland taste and aroma. It usually contains about 75-80% protein, 6-8% native lipids, fibre, residual starch, a small amount of mineral matter and between 4 and 12% residual moisture. Gluten, per se, is generally considered to consist, in approximately equal amounts, of the proteins gliadin and glutenin. Commercially, vital wheat gluten is presently manufactured by one of several washing processes in which wheat flour is kneaded with water to remove the starch and water-soluble materials from the gluten, the latter usually being obtained as a tough, rubbery, elastic mass containing a high proportion of water (about 67% by weight). If a dry product is required, this water must be removed without exposing the gluten to excessively high temperatures (generally less than about 70.degree. C.) since gluten can be denatured by exposure to higher temperatures, especially when in the wet state. Two drying processes in common usage are spray drying and ring drying.
Vital wheat gluten is an important item of commerce finding very many uses, especially in the manufacture of yeast-leavened bakery products such as bread, rolls and buns. In this application it supplements the natural flour protein where added strength is needed, or desired, for several reasons, say to build stronger sidewalls in expanded white bread or in the production of variety breads where the non-flour proteins dilute the flour protein to such an extent that the latter protein cannot function satisfactorily without supplementation. The utilization of conventional dry vital wheat gluten in many of these applications is hindered by some of its native characteristics. For example, in limited-moisture environments. e.g. pasta, it does not readily disperse and blend with the native wheat proteins to form the desired all encompassing matrix. On the other hand, in dilute aqueous media, e.g. batters or "brews" it rapidly coalesces producing non-uniform dispersions. Efforts have been made in the past to overcome, or to improve upon the poor dispersibility, and the tendency to coalesce when dispersed, of vital wheat gluten whilst ensuring that its vitality is not adversely affected. Some of these attempts have included treating gluten, or wheat flour containing gluten, with various lipids, for example, U.S. Pat. No. 3,362,829 to Landfried teaches coating dry, powdered gluten, via a melt process, with an amount of certain selected non-ionic hydrophilic lipids. The process is essentially a physical process, i.e. no chemical reaction or complex occurs, as is indicated, for example, by the fact that the obtainment of the desired results is dependent on the particle size (i.e. surface area) of the gluten being treated.
A later U.S. Pat. No. 3,880,824 to Rao et al acknowledges the Landfried patent and states that the coalescence problems associated with gluten may be solved by "reacting" dry, powdered gluten in an inert diluent, which may be any non-aqueous, preferably organic solvent, with an ionic or non-ionic lipid selected from the group consisting of fatty acid chlorides, fatty monoglycerides, lactylic esters of fatty acids, sorbitan fatty acid esters and phospholipids, the resulting product being claimed to be a "gluten-lipid complex". The specific examples show that, following "reaction", the solvent is evaporated and the fat content of the residue evaluated by a standard acid hydrolysis method. Since this method evaluates total fat, i.e. bound and unbound fat in the residue, this test alone cannot give any indication that the added lipid actually complexes with the gluten: as will be discussed later, separate evaluation of free and total lipid is necessary to determine if the added lipid has actually reacted (complexed) with the gluten. In fact, evaluation in this latter manner by the present applicant clearly shows that no complexing occurs between the gluten and the lipid under the given conditions.
However, as in the previous case, the presence of the dispersed lipid would give some improvement in the dispersibility of the product in aqueous media.
Yet a further attempt to provide a modified gluten having improved dispersibility characteristics is detailed in U.S. Pat. No. 4,035,519 to Maher M. Abou-Guendia. This patent refers to the above-described Landfried-type coating of the gluten with lipid and notes that using that procedure, uniform distribution of the emulsifier throughout the gluten is difficult. The disclosed process comprises mixing a melted hydrophilic and/or lipophilic emulsifier with wet gluten having a moisture content &gt;45% at a temperature of between 40.degree. C. and its denaturization temperature, the product optionally being dried in a known manner. Reference to the specific examples shows that mixing is effected until the emulsifier is uniformly distributed throughout the gluten and this is accomplished in a short time period of two minutes at low mixing speed. The specification refers only to an emulsified gluten which is effectively a mixture of the lipid and gluten involving, as present applicant's work shows, no significant lipid gluten reaction. As in the earlier-discussed patent specifications, acceptable utility in baking is claimed.
In other instances, attempts have been made to proceed directly from the wheat flour to a gluten product which is modified during the extraction process, the product claimed to have advantageous properties compared with gluten obtained by traditional methods. U.S. Pat. No. 3,783,139 discloses such an attempt, and in particular, a process for separating wheat flour into its starch and gluten components by forming the normal wheat dough in the presence of selected non-ionic surfactants. The process is claimed to use much less wash water than the known methods, and provide increased protein yields in the form of a modified gluten product. The text emphasizes the critical nature of the non-ionic emulsifier providing data detailing the failure of other emulsifiers to be effective.
While gluten's resilient, chewy, irreversibly gelled texture when cooked would be desirable in several significant non-bakery applications, e.g. meat and cheese analogues and extenders, confectionery gels and sausage casings, its use in such applications has been hindered by, when such applications are considered, undesirable characteristics, for example:
(a) A group of characteristics which dictate the conditions required to develop the structure/texture of conventional gluten, these conditions including, available moisture; work input (taking into account the extent, and type of, mixing); fat content and the presence of significant amounts of apparently incompatable substances such as soy protein.
(b) A group of characteristics which relate to the handling properties of the resilient gluten once formed. These characteristics include:
(i) work hardening-gluten as it is mechanically worked becomes firmer until a maximum is reached, continued working resulting in a breakdown of the gluten structure; PA1 (ii) thermosetting-gluten coagulates when heated at temperatures above 60.degree. C. for a period of time; and PA1 (iii) a high viscosity/rubbery nature.
Notwithstanding the difficulties, many attempts have been made to use gluten in such non-baking applications. Refer for example to U.S. Pat. No. 3,682,661 to Albin F. Turbak which discloses the use of edible gluten films to be used as coverings, such as sausage skins for emulsion meat products. However, attempts such as this have not been successful primarily because the resulting gluten films are found to be lacking in the strength and maleability required, especially as regards their ability to be self-supporting and, consequently, supplementation with other film-forming materials such as collagen has been necessary and/or processing conditions must be very severe.